Cable video systems often employ trap or notch filters to prevent a pay channel from reaching nonsubscribers or to descramble channels using interfering carrier scrambling signals. These filters block the particular pay channel but allow the other channels to pass. Due to the inductance/capacitance (LC) networks used in the filter devices, temperature variations produce a change in the capacitance which alters the performance of the filter circuit. This variation with temperature is further compounded by the outdoor use of such trap filters when employed in cable television systems.
It has therefore become necessary to use temperature componsating capacitors in the poles of the filter circuit. However, the design of a filter circuit with properly compensated trap filters has been a very time consuming and labor intensive process.
Standard temperature compensating capacitors are generally available in three (3) compensation values designed by an "N" (negative compensation) number. The typical N values are 0, 75, and 150. The N value denotes the temperature coefficient range expressed in parts per million per degree centigrade (ppm/.degree.C.).
Prior design processes for trap filters begin by first determining the electrical parameters for proper trap operation. Next, the components of the trap filter are compensated for temperature variations which occur in the outdoor use of the filters. The compensating step includes a search for a proper coil (L) and capacitor (C) combination in each of the filter poles that achieves the desired temperature compensation. However, because standard ceramic capacitors used in trap filters are only available in limited temperature compensations, i.e. NPO, N75 and N150, the compensation process for the trap filter circuit almost becomes a trial and error approach, or at best a poor compromise.
Further, during the compensation process several negative results occur. These negative results often include the creating of a lopsided notch due to the series pole becoming unsymmetrical with respect to the shunt, i.e. ground pole; and the lopsided notch causing interference with adjacent frequencies in the cable television system and further causing the trap filter to lose some of its stability. Overall in the production and manufacturing of trap filters, the circuit compensation is never perfectly achieved.
Thus there is a need for a highly stable and symmetrical trap filter with precise temperature compensation. Further, the design of such a temperature compensating trap filter must be possible using standard ceramic capacitors having a limited range of temperature compensating values. There also is a need for a design process which can be accomplished without excess time delays or manpower allotments.